Polycrystalline silicon nitride (Si3N4) bodies are becoming increasingly attractive as structural and mechanical materials due to their ability to provide high strength and durability under severe conditions, and especially under high temperature applications. Si3N4 is characterized by high heat resistance, mechanical strength, thermal shock resistance, wear resistance, chemical stability, and hardness. One reason that Si3N4 does not enjoy wider use is that Si3N4 green powder compacts or green bodies are inherently difficult to sinter.
Known processes for producing a sintered silicon nitride body typically require the use of a sintering aid, such as Y2O3, Al2O3, MgO, or the like, added to a raw material powder of silicon nitride and a high-pressure N2 atmosphere or N2/inert gas atmosphere under which the Si3N4 body is sintered. Typically, the sintering aids form a grain boundary liquid during sintering, and more typically this liquid includes SiO2, either contributed as an impurity present in the Si3N4 or generated by the oxidation of Si3N4 by oxygen present in the sintering atmosphere.
The grain boundary liquid thus serves as a sintering aid and forms a silica-based glass in grain boundaries. This glass aids in the densification of the silicon nitride powder green body and in the formation of a fine grain structure in the resulting sintered body. However, the relative amounts of O and N in the glass phase is an uncontrolled variable varies, and thus the composition of the glassy phase at the grain boundary is likewise uncontrolled and variable, resulting in density gradients in the sintered body and compositional gradients at the grain boundaries.
Various additives have been added to improve the mechanical strength of the sintered ceramic bodies to enable them to perform under severe conditions. Silicon carbide (SiC) has been found to provide increased resistance to oxidation and mechanical strength at high temperatures to Si3N4. However, sintered ceramics composites produced as described above from a mixture of silicon nitride and silicon carbide powders typically contain silicon carbide particles on the order of microns only in the grain boundaries of silicon nitride particles. Attempts have been made to prevent the segregation of SiC in the grain boundaries of the sintered body. For instance, a composite sintered body of silicon nitride and silicon carbide was made by mixing a silicon nitride powder with a fine silicon carbide powder having an average diameter of 0.03 .mu.m and a specific surface area of 30 m.sup.2/g and an yttria to form a green body which was sintered at 1750-1900° C. in a pressurized nitrogen atmosphere of 1 MPa; the body was further subjected it to an HIP treatment at 1750° C. in a 100 MPa nitrogen atmosphere. However, such a technique requires a bimodal PSD in the starting mixture of Si3N4 and SIC powders, and thus it is impossible to achieve a uniform mixture, resulting in a sintered body with an uneven grain structure. Further, the bimodal PSD of the main constituent powders makes an even, uniform distribution of the yttria sintering aid unlikely, resulting in insufficient sinterability and poor mechanical strength in the resultant sintered body.
Another technique for the production of a Si3N4/SiC body involves mixing a silicon metal powder, a silicon carbide powder and a sintering aid powder, forming the mixture into a green body, sintering the green body in a nitrogen atmosphere to react the metallic silicon with nitrogen to form Si3N4 which functions to bond SiC particles, and then elevating the temperature to further sinter the body via the sintering aid. Because metallic silicon is used instead of a silicon nitride powder as a starting material, shrinkage during sintering is minimized. However, this technique suffers from the difficulty in uniformly nitriding the silicon metal from the surface to the core of the body, which typically results in at least some silicon metal unreacted inside the resulting sintered body.
Yet another technique involves heat-treating a mixture of an organosilicon polymer and silicon powder in a non-oxidizing atmosphere, such as N2, and pulverizing it to form a silicacious powder characterized by a surface covered with an amorphous material consisting of silicon and carbon. The powder is formed into a green body and sintered in an N2 atmosphere. However, as with the previously-descried technique, it is difficult to uniformly nitride the body from surface to interior.
Partially crystalline composite powders of silicon nitride and silicon carbide have been produced as starting materials, mixed with a sintering aid powder and formed into green bodies which have been heated to the 1400-1600 degree Celsius range for a first sintering/reaction step and then liquid phase sintered in the 1600-2300 degree Celsius range. However, the resulting sintered body typically suffers from the generation of pores and the deterioration of mechanical properties by decomposition of amorphous components. Further, full density cannot be achieved via this technique absent elevated gas-pressure during sintering. Also, since a sintering aid powder is mixed with the partially crystalline composite powder and then sintered, the dispersion of the sintering aid powder is typically uneven, resulting in segregation of the sintering aid and inconsistent density and other physical properties observed in the sintered body.
Finally, a process of manufacturing a composite powder for the manufacture of a composite sintered body of silicon nitride and silicon carbide includes the steps of mixing silicon metal powder and carbonaceous powder together, heating the resultant mixture in an inert gas atmosphere, such as nitrogen, at a temperature of 1,400 degrees Celsius to simultaneously carbonize and nitride the silicon metal powder. However, this technique suffers from the preferential formation of β-silicon nitride, making it difficult to increase the percentage of α-silicon nitride in the composite powder. Since β-silicon nitride tends to grow in a needle shape, the resultant powder is suffers from the anisotropic particle shapes and is thus difficult to compact or pulverize.
There thus remains a need for a technique for evenly sintering a Si3N4/SiC body to density that does not require a pressurized nitrogen atmosphere and/or excessively high firing temperatures, as both requirements greatly increase the expense of the process and, thus, the end product. The present invention addresses this need.